A construction machine, such as a wheel loader, performs so-called loading work, and frequently performs an operation of raising a boom connected with a bucket to a predetermined height during the work. For example, in order to transfer a bucket on which a product is loaded to a specific position, such as an upper side of a cargo box of a truck during loading work, the boom connected with the bucket is raised to a position higher than that of the cargo box of the truck.
When the bucket is raised to a position lower than the specific position, there exists a risk of collision between the bucket and the cargo box to damage the bucket. Further, in a case where the boom is raised as described above, the boom is generally controlled at a maximum speed for work efficiency.
FIG. 1 is a schematic diagram of a wheel loader illustrated based on a front working device in the related art. According to FIG. 1, a wheel loader 100 includes a front working device consisting of a boom 20 having one end connected to a vehicle body frame 110 and driven by a boom cylinder 22, and a bucket 30 connected to a distal end of the boom 20 and driven by a bucket cylinder 32.
For example, force applied to the boom cylinder 22 may be expressed as Equation 1 below based on the illustration of FIG. 1.P=W×L1/L2 cos θ  Equation
Here, P means force applied to the boom cylinder 22, W means external force applied to a center point A of the bucket 30, L1 is a length of A-C, L2 is a length of B-C, and θ means an angle of B-D-E.
As described above, the force P applied to the boom cylinder 22 is a function related to an angle between the boom cylinder 22 (for example, B-D of FIG. 1) and a vertical wall (for example, D-E of FIG. 1) of the vehicle body frame 10 connected with the boom cylinder 22, and according to Equation 1, the force applied to the boom cylinder 22 when the boom 20 is raised is further increased.
Accordingly, in a case where the boom cylinder 22 is expanded for loading work, the boom cylinder 22 is generally expanded at a maximum speed to increase work efficiency.
Further, the wheel loader in the related art generally includes a pair of left and right oil pressure pumps in order to drive the front working device, and the pair of left and right oil pressure pumps may supply hydraulic oil to the front working device, such as the boom cylinder, through a so-called confluent circuit.
Further, a predetermined cutoff pressure is set in each oil pressure pump. As a result, the cutoff pressure limits, for example, a pressure range of the boom cylinder, that is, a maximum rising pressure range of the boom, and determines maximum force generable by each oil pressure pump.
For example, as illustrated in FIG. 2, during the loading work of the wheel loader, when a joystick for operating the boom is maximally operated, a raising speed of the boom may be maintained at a maximum speed until the boom approaches a maximum height. As represented in Equation 1, since the force (pressure, P) applied to the boom cylinder is in proportion to a raising height of the boom, a predetermined pressure value corresponding to the maximum height of the boom is set as each cutoff pressure in each oil pressure pump. Accordingly, the boom 20 may be raised to the maximum height of the boom at the maximum speed.
However, as illustrated in FIG. 2, the pair of left and right oil pressure pumps simultaneously approach the cutoff pressure in a state where the raising speed of the boom is largest, so that an increase movement of the boom is temporarily stopped, and the boom, which is raised at the maximum speed, is temporarily stopped, and thus impact may be generated and an impulse may be transferred to the bucket connected to the other end of the boom.
Since the loading work is generally to load a product on the bucket and load the product on a cargo box of a truck, and the like, when the impulse is transferred to the bucket, the product loaded on the bucket may be lost. Further, when the boom cylinder is abruptly halted during the expansion of the boom cylinder at the maximum speed, stress due to the impact may also be accumulated to a whole machine including the boom cylinder.
In order to solve the problem, technology of tracking a position (height) of the boom and controlling a rise of the boom by an electronic control method has been developed. However, for the technology, additional problems, such as complexity of a manufacturing process including a necessity to additionally include required electronic constituent elements, and an increase in cost, may be incurred, and further, a new problem, such as an occurrence of failure by the additionally included electronic constituent elements, may be incurred.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.